1. Field of Invention
This invention relates generally to techniques for trimming the ohmic values of resistors incorporated in microelectronic chips, and more particularly to a system for this purpose adapted to effect an increase or decrease in the value of a resistor without changing its physical dimensions.
2. Status of Prior Art
Microelectronics is that branch of the electronics art which deals with extremely small components, assemblies or systems. In one well-known form of microelectronic structure, resistors, capacitors and conductors are formed by depositing chemical materials onto the surface of a substrate to define a "thin-film" circuit. In another form, a substrate is also employed, but resistors and conductors are printed onto its surface, all other circuit components, such as capacitors, diodes, etc., being discrete elements. This type of microelectronic structure is known as a "thick-film" or a ceramic printed circuit.
Ceramic printed circuits are the main concern of the present invention, for these may be inexpensively mass produced, and, because of their compactness, light weight and low cost, they are widely used in many forms of modern electronic equipment. In the fabrication of ceramic printed circuits, the circuit pattern is printed on a high resolution metal screen. In separate operations, the conductor and resistor materials are pressed through the screen onto a wafer-thin substrate of alumina or other ceramic. The resistive materials are generally in the form of carbon particles dispersed in a binder solution. Use is also made of such resistive materials in particulate form as nichrome, tin oxide, cermet and titanium.
After the conductor and resistor patterns have been printed, the ceramic wafer is placed first in a low-temperature oven which dries the pattern, and then in a high-temperature furnace which fixes the resistor and conductor patterns on the substrate. Next, the conductors are dip-soldered and additional components, such as transistors and capacitors, are soldered, welded or bonded to the substrate. In a final step, the substrate is encapsulated.
While this fabrication technique gives rise to resistance values which are fairly close to the required tolerances, it is still necessary to make a final adjustment, for it is not possible to lay down precision resistors. With existing trimming methods, one percent tolerance is achievable by the physical removal of resistive material embedded in the resistor deposit following the firing cycle. Removal of this material from the edge of the printed resistor by an air-operated abrasion unit gives positive control of precision resistance values.
Nevertheless, the abrasion technique for trimming resistors has many serious drawbacks, for it not only degrades or destroys the physical qualities of the resistors, but it also reduces their physical dimensions, with an accompanying loss in power-handling capacity. Moreover, the abrasion technique is capable only of effecting an increase in resistance value so that if the resistor value, as printed, is initially too high, it is not correctable and the resistor must be rejected.
In projecting a jet of sand or other abrasive material against the resistor surface, it is difficult to control the degree of attrition, as a consequence of which the ohmic value may be caused to rise beyond the desired tolerance. Since correction can only be effected unidirectionally, in the event the trimming action overshoots the desired value, the resistor is no longer correctable and must be rejected. Thus, printed resistors which initially are too high in value or which have been excessively trimmed are beyond correction with existing abrasion trimming techniques.
A single defective resistor in a ceramic printed circuit renders the entire circuit unacceptable, and a mistake in trimming one resistor in a printed circuit assembly makes it necessary to reject the entire circuit. The likelihood of a single error is particularly great when the assembly includes a large number of resistors such as in a ladder network. In practice, therefore, with existing abrasion trimming techniques, the rejection rate is quite high. This factor raises manufacturing costs substantially.
My prior U.S. Pat. Nos. 3,676,633 and 3,647,684 (hereinafter referred to as the Di Mino patents) disclose an electronic technique for trimming the ohmic value of a resistor included in a microelectronic circuit to effect a correction in either direction with respect to the initial value of the resistor without, however, changing its physical dimensions.
Apparatus for this purpose disclosed in these Di Mino patents includes a high-frequency oscillator having a resonator coil to produce an R-F carrier, the oscillator being modulated by an audio-frequency signal to create pulsatory R-F energy. The resonator coil is inductively coupled to a step-up coil connected to a "Down" probe which produces a corona discharge beam that when directed to a point on the resistor acts to reduce its ohmic value. The resonator coil is also inductively coupled to a step-down coil connected to an "Up" probe which when brought into physical contact with a point on the resistor produces a current flow therein that acts to increase the value of the resistor. The extent of such ohmic change is determined by the duration of the treatment and by the area of the resistor subjected to treatment.
Among the advantages of electronic trimming in the manner taught by the Di Mino patents is that no mechanical grinding action is required; hence the ohmic value of the resistor is altered without degrading its physical properties or reducing its power handling capacity. The electronic trimming procedure, because it involves low-power R-F energy, produces no carbon dust or sand; hence its use is in no way hazardous to the health or safety of the operator. And because the electronic trimming technique neither increases nor decreases the physical dimensions of the resistor, it becomes feasible to correct the value of relatively delicate resistors of the type otherwise damaged or destroyed when subjected to abrasion trimming.
Nevertheless, the apparatus disclosed in the Di Mino patents suffers from one practical drawback, and this militates against its commercial use on a production line for trimming resistors.
As pointed out previously, the Up and Down probes are connected to coils inductively coupled to the resonator coil of the oscillator. It is therefore necessary to bring the chip containing the resistor to be trimmed into close proximity with the probes, for it is not possible to bring the probes by way of extension cables to the chip. The reason for interdicting the use of extension cables is that these cables, since they would be connected to coils inductively coupled to the resonator coil of the oscillator, would then act as radiating antennas. As a consequence, the pulsed radio-frequency energy would be dissipated and not be conveyed to the probes; hence this extension cable arrangement would be ineffective as an electronic trimmer for resistors.
At the time the Di Mino patents were granted, the dimensions of a typical resistor network chip or wafer containing resistors that required trimming was approximately one square inch, and the only way one could trim these resistors was to bring the chip into the close proximity of the probes to perform the required trimming operations. Currently, however, the typical resistor network chip is greatly reduced in size and is in many cases about a tenth of the size of the chips previously produced. One cannot as a practical matter handle these tiny chips to effect trimming by the apparatus disclosed in the Di Mino patents.
Modern production procedures make use of servo mechanisms or X-Y positioning tables to position a device to be worked on relative to a tool or other apparatus to carry out the work. Apparatus of the type disclosed in the Di Mino patents does not lend itself to incorporation in a servo mechanism or an X-Y positioning table, for the apparatus is relatively massive and cannot readily be manipulated.